1. Field of the Invention
The present invention relates, in a radio transmission system, to a radio terminal equipment for receiving radio waves concurrently reached from a plurality of radio stations to process the radio waves under a procedure of channel controls.
2. Description of the Related Art
The CDMA (Code Division Multiple Access) system is a multiple access system which essentially has confidentiality and an interference resistibility, and is capable of suppressing a cochannel interference and reusing a radio frequency efficiently. In addition, the CDMA system is positively being applied to a mobile communication system recently due to the fact that the establishment of technology for realizing the transmitting power control with high accuracy and high response has enabled flexible control of the radio transmission characteristics by the sector zone.
FIG. 9 is a diagram showing an example of a configuration example of a mobile communication system to which the CDMA system is applied.
In FIG. 9, two radio base stations (BTS) 71-1 and 71-2 are connected through a not-shown communication link to form wireless zones 72-1 and 72-2, respectively. A mobile station (MS) 73-1 is located in the overlapping region of the wireless zones 72-1 and 72-2, and mobile stations (MS) 73-2 to 73-4 are located in the region of the wireless zone 72-2, where the wireless zone 72-1 is not overlapped.
FIG. 10 is a diagram showing a configuration of the mobile station.
In FIG. 10, the mobile station 73-1 is composed of: an antenna 81-1; an antenna interface part 82-1 having an antenna terminal connected with the feeding point of the antenna 81-1; a transmission information generating part 83-1 fed with the “upward transmission information” to be transmitted to the radio base stations 71-1 and 71-2; a spreading part 84-1, an orthogonal modulation part 85-1 and a transmission part 86-1 connected in tandem between the output of the transmission information generating part 83-1 and the input of the antenna interface part 82-1; a receiving part 87-1, an orthogonal demodulation part 88-1 and a despreading part 89-1 connected in tandem with the output of the antenna interface part 82-1; a demodulating part 90-1 disposed at a subsequent stage of the despreading part 89-1 for outputting downward transmission information; a receiving timing detecting part (RTDP)91-1 connected together with the despreading part 89-1 directly with the output of the orthogonal demodulation part 88-1; a CDMA controlling part 92-1 connected in tandem with the receiving timing detecting part 91-1 and having one output connected with one control input of the spreading part 84-1; and a receiving signal processing part 93-1 having: a first input connected with the control input of the despreading part 89-1 and the other output of the CDMA controlling part 92-1; second and third inputs connected with the outputs of the despreading part 89-1 and the demodulating part 90-1, respectively; a fourth input connected with the input of the CDMA controlling part 92-1 and the output of the receiving timing detecting part 91-1; and four outputs connected individually with the control inputs of the transmission information generating part 83-1, the orthogonal modulation part 85-1 and the transmission part 86-1 and the other control input of the spreading part 84-1.
Here, the configurations of the mobile stations 73-2 to 73-4 are identical to that of the mobile station 73-1. In the following, therefore, the corresponding components will be designated to the common reference numerals subscribed by the suffixes “2” to “4”, and their description and illustration will be omitted.
In the mobile communication system thus configured, the radio base stations 71-1 and 71-2 transmit the transmission wave signals P1 and P2, as modulated in the series of the following frames (hereinafter referred to as the “downward frames”), as shown in FIG. 11(1) and (2), individually to specific radio channels:
Frames containing not only frame numbers SFN having values to be sequentially updated recyclically to “0”, “1”, - - - , “36863”, “8”, - - - but also broadcasting information and other control information to be applied for the channel control; and
Frames having a constant word length.
Here, the aforementioned word length is assumed to be 10 milliseconds (as will be expressed by the “frame length TF”) under a predetermined transmission speed.
In the following, on the other hand, the aforementioned transmission wave signals P1 and P2 and specific radio channels will be called the “broadcasting signal” and the “control channel”, respectively.
It is further assumed for simplicity that the “broadcasting signal” be transmitted with a specific transmission power by the radio base stations 71-1 and 71-2 but not an object of the later-described transmission power control.
Moreover, it will be assumed for simplicity that the aforementioned specific radio channel be the “perch channel” which is acquired either by the mobile stations located at the wireless zones 72-1 and 72-2 on the basis of the procedure prescribed for the system or as the information contained in the broadcasting signal.
In the mobile station 73-1, on the other hand, even for the period for which a completed call of the local station remain existing and for which speech signals or the like are transmitted/received through any radio channel formed between the mobile station 73-1 and the radio base station 71-1, for example, the receiving part 87-1 transforms the broadcasting signal P2 (FIG. 11(3)) having reached to the antenna 81-1 from the radio base station 71-2 and given through the antenna interface part 82-1 into a predetermined intermediate frequency signal.
Here, the associated operations of the individual parts concerned with the aforementioned transmission/reception of the speech signal have no relation to the invention, and their detailed description will be omitted.
The orthogonal demodulation part 88-1 generates two baseband signals orthogonal in the baseband region by demodulating the intermediate frequency signal orthogonally. The despreading part 89-1 despreads those baseband signals in accordance with a predetermined despreading code. The demodulating part 90-1 recovers the “downward transmission information” by demodulating the signal, which has been despread to the transmission band in the despreading procedure. On the other hand, the receiving timing detecting part 91-1 reproduces the aforementioned frame series.
The receiving signal processing part 93-1 synchronizes with the frames contained in that “downward transmission information” and extracts the frame numbers SFN contained individually in those frames. Moreover, the receiving signal processing part 93-1 calculates the difference between the current time and the time at which the SFN is at “0”, based on the frame numbers SFN thus extracted, and acquires and holds the point t0 of the beginning of a frame having the SFN “0”.
On the other hand, the CDMA controlling part 92-1 leads the channel controlling processing to control the operations of the despreading part 89-1 and the spreading part 84-1 under the channeling control and to control the operations of the transmission information generating part 83-1, the spreading part 84-1, the orthogonal modulation part 85-1 and the transmission part 86-1 in association with the receiving signal processing part 93-1.
Of the items relating to such channel control, here will be omitted the description of the item having no relation to the invention.
Further, the processings to be done in the transmission information generating part 83-1, the spreading part 84, the orthogonal modulation part 85-1 and the transmission part 86-1 are individually reversible ones with respect to the aforementioned processings which are done in the demodulating part 90-1, the despreading part 89-1, the orthogonal demodulation part 88-1 and the receiving part 87-1, and their detailed description will be omitted.
The CDMA controlling part 92-1 suitably transmits a transmission signal TX1 modulated with the series of the frame (hereinafter referred to as the “upward frames”) containing a later-described timing compensation value TDHO, as shown in FIG. 11(4), for the period while the local station is located in the region where the wireless zone 72-1 and the wireless zone 72-2 overlap and while the aforementioned completed call exists.
Here, the timing compensation value TDHO is given by the following formula for a point tt, at which the transmission of the frames containing the SFN “0” as the aforementioned transmission signal TX1 is started, and the aforementioned point t0:TDHO=tt−t0.
Here, the detail of the processing for the CDMA controlling part 92-1 to decide the instant at which such transmission signal TX1 is to be transmitted is omitted for simplicity and will be described hereinafter.
Further, the aforementioned downward frames and upward frames are configured, actually, as a series of time slots in a manner that the corresponding frame is divided into sixteen according to a predetermined form, as shown in FIG. 12. Here in the following, the individual time slots composing the downward frame will be called the “downward slots”, and the individual time slots composing the upward frame will be called the “upward slots”.
Here, the timing compensation value TDHO, as expressed by the aforementioned formula, signifies “the phase difference between the broadcasting signal P2 having reached the mobile station 73-1 from the radio base station 71-2 and the aforementioned transmission signal TX1” on a time axis.
Therefore, the phase difference that is the sum between the phase difference, corresponding to the difference propagation between a delay time D1 of the radio transmission line from the radio base station 71-2 to the mobile station 73-1 and a delay time D2 of the radio transmission line from the radio base station 71-2 to that mobile station 73-1, and the phase difference of the transmission waves of the radio base stations 71-1 and 71-2 is given by the following formula, when a difference d of the phases on the time axis of the frames to be transmitted in an up link (leading from the mobile station 73-1 to the radio base stations 71-1 and 71-2) and a down link (leading from the radio base stations 71-1 and 71-2 to the mobile station 73-11) on those radio transmission lines is known:δ=TDHO−d. 
Here, it is assumed for simplicity that the aforementioned phase difference d be at a value of “1,024” times of the chip rate.
The radio base station 71-1 receives the aforementioned transmission signal TX1, and extracts, the timing compensation value TDHO contained in the transmission signal TX1, for the period while the soft hand-over is to be performed on the corresponding completed call, to inform the radio base station 71-2 the timing compensation value TDHO (containing the corresponding call identification information) through the aforementioned communication link.
For the call which is given such timing compensation value TDHO through the communication link, the radio base station 71-2 maintains the orthogonality of the spreading code to be applied for transmitting, by round the deviation of the phase signified by that timing compensation value TDHO, in the accuracy of the unit period of the symbol. Moreover, the radio base station 71-2 sets, at a point in time when the aforementioned phase difference δ is suppressed in the accuracy of the timing compensation value tDHO (nearly equal to TDHO) obtained as a result of the rounding, the phase (or a point in time at which the transmission is to be started) of the frames to be subsequently sent to the radio channel to be applied for the soft hand-over.
Therefore, to the mobile station 73-1, frames (or time slots) of the speech signal reach almost concurrently from the two radio base stations 71-1 and 71-2 in with a deviation less than the aforementioned symbol period on the time axis as shown in FIGS. 13 (1) and (2).
In the mobile station 73-1, the receiving part 87-1, the orthogonal demodulation part 88-1, the despreading part 89-1, the demodulation part 90-1 and the receiving signal processing part 93-1 receive the frames that concurrently reach in parallel under the control of the CDMA controlling part 92-1.
In the mobile station 73-1, the receiving timing detecting part 91-1 detects the field strength level of the downward slots, which are received from the radio base stations 71-1 and 71-2, as described above, and decides the specific reference point according to a predetermined algorithm. The CDMA controlling part 92-1 specifies the point in time, which is determined relative to the reference point, as the transmission point.
Here, the procedure and an operand of the operations to be done for thus determining the transmission point are not the feature of the invention, and a variety of known techniques can be applied thereto so that their description will be omitted.
Moreover, the CDMA controlling part 92-1 starts, in association with the receiving signal processing part 93-1 and the spreading part 84-1, the transmissions of the upward slots (or frames) containing both or either of the speech signals to be transmitted to the radio base station 71-1 and 71-2 and any control information concerned with channel control of the corresponding call, at that transmission point, and feeds the transmission part 86-1 with the level suited for the value of the TPC bit contained in any downward slot (or frame) received in advance, thereby to control the transmission power necessary for solving the near-far problem intrinsic to the CDMA system.
Here in the aforementioned prior art, the transmission characteristics of the radio transmission line to be formed between the radio base stations 71-1 and 71-2 and the mobile station 73-1 generally vary every moment in accordance with the movement of the mobile station 73-1 and the location and the shape of the landform and planimetry intervening on the radio transmission line, and a multipath is formed. Nevertheless, the transmission point is determined relative to the reference point which was determined on the basis of the aforementioned algorithm.
When those transmission characteristics of the radio transmission line have an allowably low fluctuation, therefore, the transmission power is controlled stably and highly accurately on the basis of the aforementioned TPC bits, and a sufficient throughput is stably retained for the processing of the channel control to be made in the radio base stations 71-1 and 71-2.
In the state where the fluctuation of the transmission characteristics of the radio transmission line is serious, on the contrary, the transmission point is determined with reference to the reference point which is determined on the basis of the aforementioned algorithm, even if the downward slots that have reached in advance are received. In both or either of the radio base stations 71-1 and 71-2, it may be impossible to retain the throughput which is required for the processing to be completed before any downward slot is subsequently transmitted.
Even when the downward slot reaching after that reference point is received, on the other hand, it may be impossible that a sufficient throughput necessary for the processing of the channel control is retained for the upward slot.
Moreover, the state where the throughput is short is generally made the more liable to occur when the symbol rate is set to the smaller value.
In the prior art, therefore, it is highly probable that the effective utilization of the radio frequency suited for the amount of the transmission information to be transmitted is blocked, or that a restriction is imposed on the design of the system suited for the zone configuration, the channel allocation and another combination.